I have seen Newtonians lined with cork on the inside of the optical tube, supposedly to reduce thermals in the tube. Is this likely to be of much benefit on my Orion Optics 10" Newt? Or would it be better to fit a 12V fan below the primary mirror? Thanks!

Ideally you'd have a tube much larger in diameter than the mirror, made of a non-conductive material and have fans fitted - the fans would be more to promote rapid cooling than prevent tube currents. Tube currents can arise when there is a temperature difference within the tube, this happens because radiation cools the sky facing surfaces more than the ground facing surfaces; but they tend to stick close to the tube walls, which is why a grossly oversize tube is helpful.

I lined the interior of my 6" Newtonian (which had a 2mm thick aluminimum tube and was only about 1" oversize) using 3mm thick expanded polystrene sheet, this had a good effect. Whether it would help your scope depends on factors which are not known to me.